Union restrictions

May 4, 2023

General interfaces

Implementation restriction: A union (with more than one term) cannot contain the predeclared identifier comparable or interfaces that specify methods, or embed comparable or interfaces that specify methods.

Or putting this another way: Unions (that is, interface type parameters with a | character), only work on interfaces defined by other type parameters.

interface {
  int | float32    // Valid
  int | error      // Invalid: error is an interface that specifies a method
  int | comparable // Invalid: comparable is not a type parameter-defined interface

Quotes from _The Go Programming Language Specification_ Version of December 15, 2022
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